In the electrolytic refining of copper, the anode mud or sludge contains variable amounts of silver, gold, platinum, palladium, selenium, tellurium, arsenic, antimony, copper, nickel and lead among others, which may be valuable and should be recovered.
The chief purpose of the existing anode mud recovery or work-up processes is the separation and purification particularly of the precious and other metals that are present. At present a number of different processes for recovery of precious metals from these electrochemical wastes are being used.
ln some prior art processes the anode mud or sludge is subjected to pyrometallurgical treatment in an air or flame furnace (cupellation process) and the resulting anode silver is further processed or purified electrochemically.
These processes have a number of significant disadvantages, particularly the effort, time and energy consumed in furnace melting and the production of a variety of slags which remove many noble metals, thereby requiring many recycle steps to recover them.
Newer processes combine electrochemical and pyrometallurgical process steps. Thus, for example, according to one process (u.S. Pat. No. 4,,002,544 or German Patent document DE-AS 25 43 027) a decoppered anode sludge is roasted and sulfatized at approximately 160.degree.-300.degree. C., in order to decompose the copper and silver selenides and tellurides. Only the Ag, Se, and Te are dissolved in the hot sulfuric acid, indeed about 95% of the Ag, Se, and Te pass into the sulfuric acid.
From the filtered sludge salts Ni can be removed by leaching with hot water and Au, Pt, and Pd can be obtained from the waste residue according to known processes.
From the sulfuric acid the Ag, Se, and Te are cathodically separated by electrolysis after dilution, the metal powder containing these elements is melted under an air stream (Se0.sub.2 and Te0.sub.2 are volatilized) and electrolytic recovery of the resulting anode silver leads to the desired silver metal.
These processes indeed circumvent the melting in the furnace but involve an additional electrolysis in separating of the Ag, Se, and Te. Besides a clean separation of the elements is not possible with the sulfatization and heating step.
In another process (DDR U.S. Pat. No. 146712, 1981 see also lnternational classes C22B 15/08; C22B 11/64) the anode mud, freed from Se and Te, is subjected to a nitric acid leaching to remove Ag, Cu, and Pb, whereby Ag, Cu, and Pb go into solution as nitrates and are further processed by an electrolysis in which silver is deposited, while from the mud residue according to one of the known pyrometallurgical or hydrometallurgical processes the entire Au, Pt, and Pd content can be extracted.
The silver contained in the nitric acid solution is separated chiefly by an electrolysis process, and the residual silver is obtained by cementation with Cu. From the desilvered solution Pb is precipitated with H.sub.2 S0.sub.4 as PbS04, filtered, and the filtrate is distilled and rectified to remove H.sub.2 S0.sub.4 and HN0.sub.3 and the crystalline slurry remaining in the distillation sump is taken up with H.sub.2 0 and processed to Cu or copper salts.
In the presence of Pd in anode this last process is not suitable, because Pd is highly soluble in nitric acid, and a separate process for removal of this precious metal must be employed. Another disadvantage is that, on account of a high Pd or Cu content, an excessively expensive electrochemical process is required.
Furthermore the direct chlorination of anode mud or sludge without prior pb separation has been suggested (See German Patent document DE-OS-21 17 513). The anode mud is mixed with dilute hydrochloric acid to make a watery paste, into which with agitation gaseous chlorine is introduced at a temperature of about 100.degree. C., in order to dissolve almost all the metals contained in the mud with the exception of silver.
The slurry must be filtered hot and also washed hot, in order to remove the bulk of the Pb as PbCl.sub.2. Subsequently extraction of the AgCl occurs with ammonia, whereby Ag is separated from the remaining accompanying materials, i.e. Sb,Sn, and Si0.sub.2. The silver is obtained from the ammoniacal solution by evaporation of the ammonia, digestion of the AgCl precipitate in an aqueous NaOH solution, and reduction of the Ag.sub.2 0 occuring by a reducing sugar to pure pulverulent silver metal, which to be made available in customary form must be remelted.
However the use of Cl.sub.2 gas in a strong HCl solution (up to 12 N, i.e. 432 g/l) creates significant corrosion of the apparatus and requires use of a reflux condenser for the HCl gas vapor. Also the separation of PbCl.sub.2 from the sludge or anode mud on a large scale in the filter press because of the large temperature drop required is scarcely attainable. A clean separation of the pb and the Ag is not in fact attained in practice with this method.